The ever increasing demand for digital transmission channels in the radio frequency (RF) band presents a potentially serious problem of spectral congestion and is likely to cause severe adjacent and co-channel interference problems. This has, in recent years, led to the investigation of a wide variety of techniques for solving the problem of spectral congestion. A modulation scheme known as minimum shift keying (MSK) addresses the spectral congestion problem effectively. MSK is a well known modulation technique to those skilled in the art. For more details on modulation and demodulation of MSK signals refer to Digital Transmission of Information by Richard E. Blahut.
In non-coherent detection of MSK modulated signals base band detection can be used. This technique uses a zero IF receiver including a quadrature demodulator for recovering the modulating digital information signal. Usually the I (in-phase) and Q (quadrature) channels are fed directly into a digital signal processing (DSP) where the demodulation and decision process is performed. The use of DSP increases the cost of the receiver and requires considerable space. As spectrally efficient as MSK has been, its wide spread use in high quantity production receivers such as pagers, has not yet been implemented due to the high cost of required DSP circuits. It is therefore clear that a need exists for a means to recover MSK signals without sacrificing receiver cost and space efficiency.